According to conventional methods for kiln drying hardwoods, the temperature of the lumber is raised to relatively high and increasing levels over a period of days until the moisture content thereof is appropriately lowered. In the case of oak, for example, a time period of 24 to 45 days is typically required. Unfortunately, the temperatures employed, e.g. from 120.degree. F. to 180.degree. F. are apt to degrade hardwood lumber by cracking, splitting, honeycombing or the like.
Less degrade can be achieved at lower temperatures and utilizing dehumidification equipment to achieve consistent results under low temperature conditions. For example, dehumificication drying can be carried on at temperatures from approximately 75.degree. F. to 120.degree. F., but unfortunately appreciably longer residence times for the lumber in the kiln are required. Unless excessive periods of time are consumed, the moisture reduction achieved in a dehumidification process is limited. Moreover, neither equalization or conditioning is practical when only the dehumidification method is used.
Multistage dehumidification processes have been proposed, for example as set forth in U.S. Pat. No. 3,939,573 to Berti wherein the temperature is raised to a small extent in the latter phase of a dehumidification process, but again the moisture reduction possible is limited.